The seasons: it’s funny that some people (including some of my friends) think that the summer means the Earth is closer to the Sun, and the winter means vice versa. This is absolutely wrong. If it was true, how we could explain the fact that when it’s winter in the northern hemisphere, it’s summer in the southern hemisphere? No, the seasons exist because Earth’s axis is today tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees. Because this tilt changes, the seasons as we know them can become exaggerated. More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters.
For half of the Earth tilted towards the Sun, one hemisphere receives more solar radiation from the Sun than the other. In the image above, it’s summer in the southern hemisphere, while it’s winter in the northern hemisphere.